Making andesite through shallow hybridization of variably enriched lithospheric mantle-derived magmas

We integrate textural and in situ compositional information from plagioclase and clinopyroxene (Cpx) phenocrysts together with groundmass compositions in early Cretaceous andesites within the Sulu belt of China to propose a new petrogenetic model for andesites. Plagioclase phenocrysts are mostly andesine; they are depleted in high field strength elements (HFSE). However, clinopyroxene (Cpx) phenocrysts are either reversely-zoned (type I) or homogeneous (type II), with the zoned Cpx divided into subtypes IA and IB. All Cpx has high Mg#, low Na2O and generally low Al2O3, with depletions in HFSE and variably high 87Sr/86Sr ratios, suggesting crystallization from enriched lithospheric mantle-derived magmas that crystallized above the Moho. The cores of type IA/IB and type II Cpx have normal major- and trace-element compositional variations and similar 87Sr/86Sr ratios to each other and to plagioclase, consistent with fractional crystallization from a common magma (magma 1). The rims of type IA and IB Cpx also have normal major- and trace-element compositional variations, but these are not as evolved as the cores, and the rims have lower 87Sr/86Sr ratios, demonstrating crystallization from an isotopically-distinct magma (magma 2). Based on modelled major and rare earth element compositions of magmas inferred to have been in equilibrium with different Cpx (± plagioclase) domains, the measured groundmass compositions can be reproduced by variable mixing between the two magmas. Our study demonstrates for the first time that andesites can be made through fractionation and shallow hybridization of variably enriched lithospheric mantle-derived magmas本研究整合了中国苏鲁带内早白垩世安山岩中斜长石、单斜辉石（Cpx）斑晶的结构与原位成分信息，以及基质成分，从而提出了全新的安山岩岩石成因模型。斜长石斑晶以中长石为主，且亏损高场强元素（HFSE）。单斜辉石（Cpx）斑晶可分为反环带型（I型）与均一型（II型），其中反环带单斜辉石进一步划分为IA与IB两个亚型。所有单斜辉石均具有高镁值（Mg#）、低氧化钠（Na₂O）及普遍偏低的氧化铝（Al₂O₃）含量，同时亏损高场强元素，且87Sr/86Sr比值存在不同程度的偏高，指示其起源于富集型岩石圈地幔源区的岩浆，并在莫霍面之上发生结晶。IA/IB型与II型单斜辉石的核部具有正常的主量与微量元素组成变化特征，且彼此之间以及与斜长石的87Sr/86Sr比值相似，表明其源自同一岩浆（岩浆1）的分离结晶作用。IA与IB型单斜辉石的边部同样具有正常的主量与微量元素组成变化，但演化程度低于核部，且87Sr/86Sr比值更低，说明其源自同位素组成迥异的另一岩浆（岩浆2）。基于与不同单斜辉石（±斜长石）结构域平衡的岩浆的模拟主量与稀土元素组成，实测的基质成分可通过两种岩浆的可变混合作用得以复现。本研究首次证实，安山岩可通过不同程度富集的岩石圈地幔源区岩浆的分离结晶与浅部混合作用形成。